We have concentrated our research efforts in three areas: 1. Isolation of ts mutants. 2. Characterization of the defective functions of two ts mutants. 3. Investigation on other biological properties of MuLV using ts mutants. 1. We have developed a quick assay for MuLV and an automatic microtitre replicate plating for cell cloning. These methods will enable us to screen a large number of clones in a short time. 2. TEM studies showed that virion is restricted after it has been initiated and that the ts lesion is in a step common to the whole assembly process. Biochemical studies by SDS-PAGE indicated that the glycoprotein of ts3 has been altered. Ts7, a new ts mutant, was found to produce defective particles at 39 degrees C. 3. Ts3 has been used to investigate the mechanism of viral induced cell fusion. Ts3 has also been used to study the ultrastructure of MuLV and the effect of interferon on MuLV replication. BIBLIOGRAPHIC REFERENCES: Wong, P.K.Y., Yuen, P.H., and Kaufmann, S.J., (1977). Induction of syncytia by Moloney leukemia virus in myoblasts defective in differentiation. J. Virol. 21, 319-327. Wong, P.K.Y., Yuen, P.H., MacLeod, R., Chang, E.H., Myers, R., and Friedman, R.M., (1977). The effect of interferon on de novo infection of Moloney murine leukemia virus. Cell, (In press).